Introduction

The heat stress induces alterations in the metabolic system (^{Baumgard and Rhoads, 2013}), which includes the decrease in the release of growth hormone and thyroid, causing a reduction in the basal metabolic rate (^{Aggarwal and Upadhyay, 2013}), affecting the expression of genes and proteins involved in the metabolism of energy and nutrients (^{Sanz et al., 2015}). Zinc in the form of divalent metal ion, Zn^2+, is nutritionally essential for all living organisms (^{Maret, 2013}); it is a trace mineral with proven importance for the function of more than 300 enzymes (^{Chasapis et al., 2012}). The metabolic action of Zn includes energy metabolism, protein synthesis, nucleic acid metabolism, epithelial tissue integrity, repair and cell division, transport and utilization of vitamin A and vitamin E absorption (^{Borah et al., 2014}).

It has been suggested that Zn is required by the fetus to support cell proliferation, and tissue differentiation in developing organs (^{Terrin et al., 2015}). Supplementation with Zn in the diet of the sow during pregnancy and lactation reduces pre-weaning mortality (^{Payne et al., 2006}; ^{Romo et al., 2017}), improves the condition of piglets during lactation (^{Caine et al., 2009}) and the immune function of piglets (Romo et al., 2017). It has also been suggested that the addition of Zn to the diet prevents reduction and improves intestinal integrity during heat stress (^{Sanz et al., 2014}), decreases intestinal permeability in piglets during weaning (^{Zhang and Guo, 2009}). ), promotes the restoration of intestinal epithelium (^{Song et al., 2015}) and improves protein metabolism in pigs (^{Pearce et al., 2015}).

Because of Zn requirements are increased during heat stress (^{Lagana et al., 2007}), it has been suggested that the Zn addition to the diet can be used to attenuate the serum Zn decrease during periods with high environmental temperatures (^{Li et al., 2015}). Diets for pigs are generally supplemented with inorganic Zn (ZnSO₄ or ZnO) to ensure the required consumption; being ZnSO₄ the inorganic source with the highest bioavailability (^{NRC, 2012}). In recent years, the use of organic sources has been explored due to its greater bioavailability (^{Star et al., 2012}).

The objective of the present study was to evaluate the effect of supplemental supplementation of organic zinc in the productive response of pigs under initiation, under conditions of high environmental heat load, born to mothers supplemented or not with organic Zn during gestation and lactation.

Material and Methods

The experiment was carried out in the pig farm "La Huerta", located in the Sindicatura de Culiacancito, Culiacán, Sinaloa., with geographic coordinates: 24o 49 '38' North latitude and 107^o 22 '47' West longitude. The place has an altitude of 60 meters above sea level (m a.s.l); the climate is classified as very warm semi-dry (BS1 (h ')), with an average annual temperature of 24.9 °C, with maximum temperatures of 45 ^oC in the months of July and August and minimums of 7 ^oC in December and January. The average rainfall is 671.4 mm, with maximum rainfall in the months of July, August and September.

The work was carried out during the months of May-July (first period) and September to November 2015 (second period); the average temperature during these periods was 28.68 °C and relative humidity 63% (CIAD, 2015). During the test period the pigs were exposed to a temperature and humidity index (THI) of 78.19 ± 2.9, according to ^{Mader et al. (2006)}.

Experimental design. 816 piglets were used with an average age of 21 days and 6,280 ± 0.817 kg of body weight, from a previous study (^{Romo et al., 2017}), carried out in sows that received or not food added with 100 mg of organic Zn from zinc methionine (MetZn) / kg of food from 35 days of gestation to the moment of weaning. The piglets were assigned to one of four treatments in an BCA´s experimental design with a 2 x 2 x 2 factorial arrangement, to receive or not, food additionally supplemented with 100 mg of Zn / kg; where the factors were: the method of supplementation (1- supplementation in pregnancy-lactation and 2- supplementation during the period of initiation), the level of additional supplementation of Zn (0 and 100 mg / kg of food) and the study period (1. May-July and 2. September-November).

The treatments were: 1) Mothers not supplemented-piglets not supplemented (control, n = 204); 2) Mothers not supplemented-supplemented piglets (ZnC, n = 204); 3) Mothers supplemented-piglets not supplemented (ZnGL, n = 204) and 4) Mothers supplemented-supplemented piglets (ZnGL + ZnC, n = 204). During the first two weeks after weaning the pigs received commercial feed, Vimifos Fase 1® and Vimifos Fase 2®, to which 100 mg Zn organic / kg was added; the pre-initiation and initiation diets were elaborated in the corn-soybean paste farm. The diets contained the nutritional contribution for each physiological stage (see Tables 3 and 4).

Management of animals. In each study period, the piglets previously weighed and identified were housed in 12 corrals, each with a space of 12 m² (8 x 1.5 m); the corral was divided with a feeder of steel type hopper to the center, in such a way that in one of the halves (4 x 1.5 m) 17 females were lodged and in the other 17 males. The corral had steel grid floor, in closed rooms totally roofed and with forced ventilation. Each of the divisions had two metallic pacifier drinking troughs. The pigs had permanent access to drinking water and food at free access. On day 49 after the start of the study the pigs were weighed. The experimental unit was the complete corral.

Measurement. The food served in each corral was registered. At the end of each test period, the pigs of each corral were weighed and with the information of feed consumption and weight gain, the average daily gain in weight and daily feed intake was obtained, as well as the feed conversion. Mortality was also recorded during each study period.

Statistical Analysis. To the variables of food consumption, daily weight gain and feed conversion, an analysis of variance was applied for a randomized complete block design with a 2 x 2 x 2 factorial arrangement. The values of number of deaths and mortality were analyzed by nonparametric statistics with the Kruskall-Wallis test. Contrast analysis was performed to determine the effect of the supplementation method on the variables under study; the contrasts were: Control vs. Treatment 2 (Mothers not supplemented-piglets supplemented) and Treatment 4 (Mothers supplemented-piglets supplemented) and Control vs. Treatment 3 (Mothers supplemented-piglets not supplemented) and Treatment 4 (Mothers supplemented-piglets supplemented). The alpha to accept statistical difference was P <0.05.

Results and Discussion

The climate conditions during the experiment are shown in Tables 1 and 2. The maximum daily THI exceeded the limit thermo neutral value for the pigs, which is from a THI of 74 (^{Mader et al., 2006}) for each day of the study. During period1 (May-July) the pigs were exposed to an average environmental temperature of 29.66 °C and relative humidity of 54.45 %; what according to ^{Mader et al. (2006)} were in an average THI of 78.49, which indicates a state of physiological alertness; however, during most of the time the experiment lasted they were at a THI higher than 80; which indicates that the pigs were exposed to heat stress. During period 2 (September-November), the pigs were exposed to an average environmental temperature of 27.7 °C and relative humidity of 70.39% (THI = 77.9, physiological alert).

In growing pigs, constant exposure to heat stress markedly increases respiration rates and body temperature, decreases body weight gains and significantly reduces feed intake (^{Pearce et al., 2013b}); likewise, it causes a redistribution of blood to the periphery in an attempt to maximize the dissipation of radiant heat; while at the gastrointestinal level vasoconstriction occurs to redefine the blood flow (^{Lambert, 2008}); consequently, the reduction of blood flow and nutrients to the intestinal epithelium compromises the integrity of the intestinal barrier (^{Yan et al., 2006}). ^{Pearce et al. (2013a)} indicated that both caloric stress and reduced food intake decrease intestinal integrity and increase the permeability to endotoxins.

Zinc is essential for the normal function of the intestinal barrier and for the regeneration of damaged intestinal epithelium (^{Zhong et al., 2010}); Zinc supplementation reduces the intestinal permeability of piglets during weaning (^{Zhang and Guo, 2009}). In addition, Zn participates in the antioxidant defense system of the animal organism and Zn deficiency increases the oxidative damage in the cell membrane, caused by free radicals (^{Waeytens et al., 2009}; ^{Wang et al., 2013}).

The effect of food intake added with Zn methionine during the gestation and lactation stage (GL), by the sow and the piglets during the initiation stage is shown in Table 5. Mortality tended to be lower (P = 0.06) in weaned pigs from sows that consumed food added with 100 mg of Zn / kg of feed during the GL stage.

In a previous work ^{Romo et al. (2017)}, observed that the additional consumption of 100 mg of Zn during the gestation and lactation period, decreased (P = 0.006) the mortality of piglets during lactation (11 vs. 26%); similar results reported ^{Payne et al. (2006)}, who offered food with 100 ppm of ZnSO₄ plus the addition of 100 ppm of an organic source of Zn-amino acids (ZnAA), from 15 d of gestation; and during lactation observed a greater survival of piglets during lactation and more piglets weaned per litter. Also, ^{Caine et al. (2009)}, reported that the consumption of diets added with 250 mg / kg of ZnAA by the sows during the last third of gestation, as well as the gastric administration by intubation of 40 mg Zn, from zinc methionine (MetZn) to the lactating piglets at the time of birth, at 7 and 14 d of age, improved the condition of the piglets during lactation.

It has been suggested that Zn is required by the fetus to support cell proliferation, and tissue differentiation in developing organs (^{Terrin et al., 2015}). These effects of Zn during gestation and lactation are especially important because the sow can provide the necessary nutrients to support the growth and development of the fetus and the piglet; in addition, the body reserves of the trace elements have been shown to serve as a source for meeting the nutritional requirements of the fetus (^{Mahan and Vallet, 1997}).

There are several reports regarding the effect of Zn in pigs, but there are few studies conducted with organic sources of Zn in pregnant and lactating sows, and the subsequent effects on the growing pig. In this regard, ^{Caine et al. (2009)} suggested that the consumption of diets added with 250 mg/kg of Zn from ZnAA, during the last trimester of gestation of the sows, raised the serum concentration of Zn in the suckling piglets, but did not report on the performance of the piglets during the initiation stage. ^{Romo et al. (2017)} indicated that piglets weaned from sows that consumed food supplemented with Zn from 35 days of gestation and during lactation had a higher (P = 0.001) plasma concentration of IgG (267 vs. 390.8 ng / ml) at 14 days post-weaning. It has also been reported that in broilers supplementation with Zn increases the IgM and IgG titers (^{Sunder et al., 2008}).

The consumption of food added with 100 mg of Zn / kg of feed, from zinc methionine, during GL and 49 days post-weaning, did not improve (p> 0.05) the productive behavior of the pigs in initiation. It is known that weaned piglets are subjected to important nutritional and environmental changes, which dramatically alter the balance of the microbiota of the gastrointestinal tract; which provides an opportunity for pathogens to colonize and cause disease, and cause poor performance in growth and even death. It has also been reported that heat stress damages the integrity of the intestinal barrier, which can increase the circulation of endotoxins (^{Pearce et al., 2012}).

It has been suggested that supplementation with Zn at the appropriate dose could improve aspects of the integrity of the small intestine and attenuate intestinal damage (^{Ineu et al., 2013}, ^{Sanz et al., 2014}). However, pharmacological levels of Zn (between 300 to 3000 ppm) are often used in the pig industry in the diets of pigs in initiation, immediately after weaning; and it has been reported that they increase growth performance (^{Morales et al., 2012}).

In this study the inorganic Zn content of the control diets used was 2064 mg of Zn / kg in the commercial food Vimifos Fase 1® and 2395 mg / kg in Vimifos Fase 2®, consumed during a period of seven days after weaning each one; in the pre-starter food that was offered during a period of 14 days, the content was 1251 mg of Zn / kg and the initiation food contained 173 mg of Zn / kg, which was offered for a period of 21 days; levels that are above the nutritional requirements of the pig (^{NRC, 2012}). To the control diets were added 100 mg of organic Zn / kg, from zinc methionine; additional level that did not improve the productive response of the pigs in the initiation stage.

Pharmacological levels of inorganic zinc prevent diarrheal diseases in piglets; in this regard it has been shown that zinc oxide has antimicrobial properties, causing changes in the gastrointestinal ecosystem of the piglet (^{Molist et al., 2011}, ^{Slade et al., 2011}, ^{Pieper et al., 2012}, ^{Hu et al., 2013}); which led to the assumption that high levels of zinc oxide in the diet, increases the growth of weaned pigs by controlling the population of bacterial pathogens at the intestinal level. In this regard, ^{Zhang and Guo (2009)} indicated that high concentrations of Zn in the diet decrease intestinal permeability by preventing the transfer of pathogenic bacteria through the intestinal barrier; in this sense, ^{Debski (2016)} suggested that for the treatment of diarrhea, only pharmacological doses of 2000-3000 mg of ZnO/kg of food are beneficial during the first 2-3 weeks after weaning; although other researchers have reported that pharmacological doses of 1000 to 3000 mg of zinc/kg of food can be administered to piglets for up to five weeks, to prevent or overcome post-weaning diarrhea and improve the performance of pigs (^{ANSES, 2013}; ^{Sales, 2013}).

It has been shown that the stress of weaning causes Zn deficiencies in piglets (^{Davin et al., 2013}), which can affect their productive behavior; however, the replenishment of these losses does not justify supplementation with pharmacological doses, because zinc homeostasis is a "closed system", and only about 0.1% of total zinc needs to be replenished daily (^{Maret and Sandstead, 2006}).; for what seems to be, that for pigs raised in environments with high sanitary status, Zn supplementation should be done at low levels. In this regard, it has been proposed in the European community that the maximum concentration of Zn in the piglet and sow feed should be 150 mg / kg of feed (^{EFSA, 2014}).

The results obtained in the productive behavior of the pigs at the beginning of the present study were predictable; however, the decrease in mortality in pigs during this stage, from sows that received diets supplemented with Zn during pregnancy and lactation, suggests that this method of supplementation may improve the immune response and physiological adaptation capacity of piglets to stressful events during the initiation period.

Conclusion

The supplementation with 100 mg of organic Zn/kg of food from zinc methionine during pregnancy and lactation can be a useful method to reduce the mortality of piglets during the initiation stage, reared under conditions of high heat load.